It is possible that it is due to the same issue as herehere, though most of the time, this would cause that the monthly reputation is actually bigger.
In your case, you have either cast a downvote on answers (which costs you 1 rep, but does not count for the monthly rep since it is a private event), or you had downvoted posts that you or the community have deleted.
The latter seems likely, since you have the Peer Pressure badge.
